The present disclosure relates to portable communication device, examples of which include mobile, or handheld, devices such as pagers, cellular phones, cellular smart-phones, wireless organizers, personal digital assistants, wirelessly enabled notebook computers, and the like; and more particularly to controlling the intensity of the wireless signals transmitted by such portable communication devices.
A wide variety of different types of portable wireless communication devices are on the market for communicating voice, data, images, and other forms of information. When being used, some of these devices are held against the ear of the user and some of the emitted radio frequency energy is absorbed by the user's body. A measurement of absorption of energy at a particular radio frequency is specified as a Specific Absorption Rate (SAR). As will be appreciated, the SAR value depends heavily upon the location of the transmitting antennas with respect to the body and the amount and the duration of the transmitted power. With a cell phone, for example, that is held against a person's ear, a greater intensity of radio frequency energy can be emitted by an antenna located near the bottom of the device positioned adjacent the jaw of the user as compared to when an antenna is located at the top of the device immediately adjacent to the ear.
Government agencies, such as the Federal Communication Commission (FCC) in the United States of America, have adopted limits for safe exposure to radio frequency (RF) energy. For example, the FCC limit for exposure from cellular telephones is a SAR level of 1.6 watts per kilogram (1.6 W/kg), which is referred to as a specific absorption rate limit.
Voice transmissions may employ a communication protocol in which the transmissions occur in a one millisecond transmission slot contained within a 20 millisecond frame, i.e., a given device transmits once every 20 milliseconds. When transmitting data, it is desirable to utilize as many of transmission slots in each frame as possible in order to send the data quickly. However, the more of the frame that is used, the greater the RF energy that is emitted and thus the specified SAR limit may be exceeded by the data transmission.
As a consequence, although the technology exists to increase the rate of data transfer, there is a concern that transmitting at a high data rate and a relatively high RF power could violate the SAR limit.